1. Field
Example embodiments relate to methods of removing residual charges, for example, in a photoconductor layer constituting an X-ray detector and X-ray imaging methods and apparatuses using the residual charge removing method.
2. Description of the Related Art
An X-ray imaging system is widely used in various fields such as industrial, scientific, and medical fields for non-destructive testing, testing of structure and properties of materials, diagnostic imaging, and to assist in performing security checks. Generally, an X-ray imaging system includes an X-ray emitter which emits an X-ray and an X-ray detector which detects an X-ray transmitted through a target object.
An X-ray emitter generally emits X-rays by colliding electrons emitted from an electron-emitting device (e.g. a cathode) to an anode. Electron-emitting devices may include hot cathode devices and cold cathode devices. A hot cathode an electrode that is heated by electric current passing through a filament while a cold cathode is an electrode that is not electrically heated by a filament but instead may rely on field emissions (FE).
In the absence of a strong electric field, electrons may need to acquire a certain minimum energy, called a work function to be emitted. In contrast, in field emission (FE) techniques, the emission of electrons may be induced by an electrostatic field. An electron emitting device utilizing field emission may be driven at a relatively low voltage. Therefore, there is research in progress for commercialization of electron-emitting devices utilizing field emission.
An x-ray system may be analog or digital. In a digital x-ray system, an image may be generated by indirectly converting photons from the x-ray into an electrical signal via visible light or directly converting the x-ray photons into the electrical signal using a photoconductor. By directly converting the x-rays into electric signals, the imaging device may generate an image with a relatively high resolution.
In digital x-rays systems, there may be residual photons held in the photoconductor within a frame which may cause a phenomenon known as an after image in which artifacts from previous X-ray exposures are visible in later X-ray images. Such undesirable image artifacts decay in a lag time which may be a limitation in high-speed x-ray machines.
Conventionally, after-images may be removed by uniformly irradiating the photoconductor with visible light to generate new charges to combine with the residual charges or applying a reverse bias voltage to the photoconductor to force the charges to recombine. However, irradiating visible light onto the photoconductor, alone, may require a relatively long period of exposure time (e.g. several dozen seconds) to effectively remove the residual charges. Further, conventional photoconductor materials have a relatively high bias voltage. For example, amorphous selenium (a-Se) may require a bias voltage of several kilovolts (kV). Therefore, applying a reverse bias voltage to a conventional photoconductor may require a relatively high bias voltage. To generate such a high bias voltage, a detector may need to include a high voltage generator which may take up a relative large portion of the detector, thus increasing the size thereof. Further, when a material having a high bias voltage is used, switching between a bias voltage and a sufficient voltage having an opposite polarity to recombine the residual charges may take a relatively long time and put unnecessary strain on the electrical components of the detector, which may cause reliability issues.